In the multi-cylinder engine as described above, control of stopping an operation of an intake valve and an exhaust valve of a cylinder to be deactivated is executed when the engine is shifted to a reduced-cylinder operation. In view of the above, the engine is provided with a valve stopping mechanism for stopping the intake valve and the exhaust valve.
For instance, Patent Literature 1 discloses a valve stopping mechanism, in which a locker arm for cam lifting and a locker arm for valve driving are adjacent to each other and supported on a locker shaft, and the locker arms are switched between a connected state and a disconnected state by a hydraulic pressure. Specifically, in a state that the locker arms are connected to each other, the locker arms integrally swing around the locker shaft, as a lift cam (camshaft) is rotated, and an intake valve and an exhaust valve are operated as the locker arm for valve driving swings. On the other hand, in a state that the locker arms are disconnected from each other, only the locker arm for cam lifting swings as the lift cam (camshaft) is rotated, and the operation of the intake valve and the exhaust valve is stopped.
In the valve stopping mechanism, when the engine is in an all-cylinder operation mode, hydraulic oil is supplied to a second oil passage in a state that supply of hydraulic oil to a first oil passage is stopped, and the locker arms are kept in a connected state. When the engine is in a cylinder deactivation mode, hydraulic oil is supplied to the first oil passage in a state that supply of hydraulic oil to the second oil passage is stopped, and the connected state between the locker arms is released.
In the aforementioned configuration, when an all-cylinder operation mode is continued for a long period of time, the amount of hydraulic oil in the first oil passage may decrease because supply of hydraulic oil to the first oil passage is stopped for a long period of time, and a response delay may occur when the engine is shifted to a cylinder deactivation mode.
Further, when the engine is left in a stopped state for a long period of time, return oil from the first oil passage may cause intrusion of air into the first oil passage, or air dissolved in hydraulic oil may turn into gas. As a result, air may stagnate in the first oil passage. In such a case, when the engine is driven next time, the valve stopping mechanism may be operated due to the stagnated air against a driver's intention, and the valve stopping mechanism may be erroneously operated.
Therefore, in an engine provided with a valve stopping mechanism, there is a demand for suppressing a phenomenon such as a response delay as described above, not to mention the disclosure of Patent Literature 1.